GPS World, March 2011

TRANSPORATION Road is a high quality L1 L2 receiver using a geodetic grade antenna used with WAAS enabled The GPS INS system was connected to the geodetic grade antenna The RF recording system was also connected to the automotivegrade GPS L1 antenna The data was collected on a test route in Detroit Michigan that included durations of urban and deep urban canyon 40 miles per hour mph or less freeway 55 70 mph and suburban local 30 mph driving The RF data were subsequently replayed to GNSS receivers that were was also replayed to receivers with forced sky visibility obstructions and various WAAS settings For limited sky visibility tests certain satellites were removed from each selection and restriction was done to mimic typical sky view obstructions encountered in normal driving Type A receiver was chosen to illustrate the impact of visibility differences A total of 13 satellites were visible in the entire data set FIGURE 4 To create obstructed sky view scenarios two Type A receivers were in their position solutions These and 13 blocked 21 and 31 blocked C1 mimics a vehicle receiver with no visibility in the Northwestern part of the sky whereas C2 mimics a receiver without visibility in the East Northeastern part of the sky Sky visibility restrictions do not vary with the heading changes of the vehicle For example for C1 receiver Northwestern sky is always obstructed regardless of the vehicle orientation FIGURE 5 shows an example RF data replay setup The record and replay system was controlled through a PC and the recorded data was also stored in the controller PC The output RF signal was split into multiple outputs such that multiple receivers can be tested at the same time For each replay of the RF data a benchmark receiver was also used to verify that there is no run to run difference as a result of the RF replay Outputs from each GPS receiver formats The recorded output from each receiver included its position position error estimate velocity indicators such as pseudorange carrier phase and signals to noise ratio Data Processing and Analysis format then corrected for antenna offsets To simplify the process the reference system position solution was translated to the position of the test antenna using the known betweenantenna distance and orientation of the vehicle as measured by the reference system As a result all the receivers and the reference system are reporting the location of the test antenna velocity for each receiver were timematched with the reference solutions and the actual error was calculated For a limited dataset additional measurement level differential processing was done to show the difference between a DPOS and an RTK or a code based differential relative position solution FIGURE 6 shows a plot of the 2D position error observed from each receiver during the test as a function of driving environment Overall Type B receiver shows better accuracy as expected from a dual frequency high quality receiver However it shows spikes of large error increases at times mostly observed in the freeway scenario with large error excursions With Type A receivers relatively larger errors are observed with the limited constellation receivers FIGURE 7 shows the number of satellites used by each receiver in the Overall Type A receiver tracks and uses on average 2 3 satellites more compared to the Type B receiver likely due to its high sensitivity capability Type A C1 and C2 receivers also track and use 2 3 satellites fewer compared to the all in view Type A receiver Freeway Data The vehicle heading in this segment was predominantly north or northwest The sky view can be considered a combination of urban and open sky conditions As highlighted in Figure 7 all in view Type A receiver was able to use up to 11 GPS satellites with an average of around 9 satellites Type A C1 and C2 receivers used on average about 3 satellites fewer than the all in view receiver All three receivers show satellite count drops down to 4 at certain times in this segment The satellite count of the Type B receiver shows the limitations of not using the high sensitive tracking capability The satellite count shows frequent drops below 4 satellites and FIGURE 4 Sky view during the test FIGURE 5 RF data replay set up GPS World March 2011 www gpsworld com 38